The power level of such spurious signals may be higher than the received signals. The spurious signals must be strongly attenuated, especially at high transmission speeds in full-duplex mode. They are mainly the result of direct leakage due to less than perfect separation between the send and receive channels of each terminal, producing a local echo. Channel separation is achieved by balanced hybrid couplers which must be matched to the characteristic impedance of the transmission channel. Separation is never perfect in that there is always a certain degree of mismatch since the characteristic impedance of the transmission channel can not be synthesised in a simple manner, varies with time and may not be known accurately (when using the public switched telephone network, for example). Spurious signals also arise from indirect leakage producing delayed echoes by reflection at impedance mismatches at various points along the transmission channel.
These spurious echo signals are eliminated by means of echo cancellers which generate a copy of the echo signal from the transmitter output signal and subtract this from the signal received at the same terminal. Adaptive echo cancellers, in which operation is controlled on the basis of an error signal derived from the receiver input signal, are often used to match the copy of the echo signal to the original echo signal.
It is known to construct an adaptive echo canceller from a time-domain transversal filter with controlled weighting coefficients and a subtractor circuit. The transversal filter intertap interval is less than the Nyquist interval of the transmitted signal (the reciprocal of twice the maximum frequency of the transmitted signal). The filter receives the output signal from the transmitter and synthesises a copy echo signal. The subtractor generates the receiver input signal by subtracting this copy echo from the received signal. The receiver input signal is also used to control the transversal filter weighting coefficients.
In this type of adaptive echo canceller, a digital transversal filter is expensive as it must operate at high speed on parameters (weighting coefficients and transmitter output signal samples) which must be encoded using a large number of digits. It has therefore been proposed to synthesise the copy echo signal from the data to be transmitted, rather than from the signal as transmitted. More specifically, the copy echo signal is synthesised from multivalent symbols substituted for the data on transmission. These symbols may be encoded using a smaller number of digits in view of the limited number of possible states.
It is standard practice in data transmission to replace the stream of data to be transmitted either by a stream of multivalent real symbols at a lower transmission rate and filtered prior to transmission on one channel, or else by a stream of pairs of multivalent real symbols likewise at a lower transmission rate and filtered prior to transmission, but transmitted on two independent channels in phase quadrature. The first of these methods is used in particular for synchronous baseband data transmission or in shifted band data transmission using single sideband or residual sideband amplitude modulation. The second method is used for passband synchronous data transmission using amplitude modulation of two carriers in phase quadrature, such as 4 or 8-state phase shift modulation or combined phase and amplitude modulation. In view of the use of two independent channels in phase quadrature, it is standard practice to consider the second method as equivalent to the first by regarding the pair of symbols as the real and imaginary parts of a single complex symbol and using the concept of complex filtering. For convenience, it will be assumed in the following description that the transmitter of carrier modulation synchronous data transmission equipment comprises a coder converting the stream of data to be transmitted into a stream of multivalent complex symbols at a reduced transmission rate, a complex signal-shaping filter and a modulator receiving two carriers in phase quadrature and, on respective inputs, the real and imaginary parts of the aforementioned complex symbols, after shaping in the filter.
Known in the prior art are echo cancellers which synthesise the copy echo signal from complex symbols output by the transmitter coder. These cancellers comprise:
a time-domain complex transversal filter with controlled weighting coefficients and an intertap interval equal to the Baud period (reciprocal of transmission rate) and receiving on respective inputs the real and imaginary parts of complex symbols output by the coder of the transmitter, with which it is synchronised,
a modulator receiving two carriers in phase quadrature and on respective inputs the signals from respective outputs of the complex transversal filter,
a subtractor providing the receiver input signal by subtracting the received signal from the modulator output signal, and
a demodulator and decision circuit in the receiver providing an error signal controlling the transversal filter weighting coefficients.
Digital implementation of the transversal filter is considerably facilitated by the restricted number of digits needed to encode the real and imaginary parts of the complex symbols constituting its input signal. The price to be paid is the use of a modulator and, above all, the need to synchronise the transmitter and receiver of the terminal incorporating the echo canceller. Synchronisation is required because the synthesised copy echo signal is incomplete, being valid only at those times at which the complex symbols appear at the output of the transmitter code. The Baud period adopted for the transversal filer intertap interval exceeds the Nyquist interval for the real and imaginary parts of the complex signals after shaping.
To eliminate the need to synchronise the transmitter and receiver of a terminal, it has been proposed that the transversal filter intertap interval should be a submultiple of the Baud period less than the Nyquist interval for the transmitted signal, that null complex signals should be applied to the input of the transversal filter at those times when complex symbols appear at the coder output, and that a demodulator independent of the receiver demodulator should be used to generate the error signal used to control the transversal filter weighting coefficients. This makes it possible to synthesise a complete copy of the echo signal, but at the price of using a demodulator separate to that of the receiver and increasing the rate at which operations must be carried out in a digital implementation of the circuit.